1. Field of the Invention
The present invention relates to a technology for compensating a voltage error caused by a dead time, and more particularly to an improved method and apparatus for compensating a voltage error caused by a dead time of a motor driving inverter which makes it possible to compensate for a distortion of output current resulting from the dead time occurring at a point in which the output current of the inverter is varied.
2. Description of the Prior Art
Referring to FIG. 1 illustrating a motor driving apparatus which employs a conventional inverter, a three-phase alternating current power source AC is rectified via six diodes D11-D16 in a converter 1 and smoothed by smoothing condensers C1, C2 which are connected in series and in parallel with the phase rectifiers, and the rectified and smoothed direct current voltage is switched and supplied to a motor IM in accordance with control signals CS1-CS6 applied to respective gates of six switching devices Q1-Q6 in an inverter 2.
A current detector 3 detects the current supplied to motor IM, and converts the detected value to a digital value which is then transferred to a control unit 4, and in accordance with the converted current value the control unit 4 generates a three-phase voltage command signal. A pulse width modulator 5 carries out a pulse width modulation of the voltage command signal and generates control signals CS1-CS6 which are then supplied to respective gates of the switching devices Q1-Q6 in the inverter 2.
Here, reference numerals D1-D6 denote freewheel diodes and are connected in parallel with each of the switching devices Q1-Q6.
With reference to FIGS. 2 through 4, a method for compensating a voltage error caused by a dead time occurring in a motor driving inverter according to a conventional art will now be described.
The power source AC rectified, smoothed and converted into a direct voltage in the converter 1 is applied to the inverter 2. The switching devices Q1-Q6 which are grouped to three pairs Q1, Q4; Q2, Q5; Q3, Q6, wherein each of the pairs are connected in parallel with each other, are alternatingly turned on/off, in accordance with control signals CS1-CS6. At this time, when two switching devices for example devices Q1 and Q4 as shown in FIG. 2A are simultaneously turned on, there is formed a current path in accordance with devices Q1 and Q4 thereby resulting in damaging the inverter 2. Therefore, as shown in FIG. 2B, in order to prevent the pair of switching devices Q1, Q2, from simultaneously turning on, whenever a switching state is changed, there is required a dead time t which denotes a simultaneous off-state of the two switching devices Q1, Q4. Here, dead time t is required to drive motor IM using inverter 2.
However, dead time t causes an error voltage between a command voltage and an output voltage to occur in the inverter 2, thereby resulting in disadvantages such as current distortion and hunting.
Conventional dead time compensating methods for a motor driving inverter rendered to solve these disadvantages include a first method in which the dead time is compensated for by applying a voltage to each end of the switching devices comprising the inverter, and a second method employing the output current of the inverter. The first method requires an additional voltage detecting device and noise may be incurred to the entire system as an undesired consequence during realization of the device, so that the second method is generally preferred.
The second method will now be described with reference to FIGS. 3 and 4.
FIG. 3 shows a flow chart for calculating a compensating voltage V.sub.D so as to compensate a voltage error in accordance with a dead time.
Current detector 3 detects (step S1) current i1 applied to motor IM as shown in FIG. 2C. Control unit 4 judges (step S2) polarity of the detected current i1. Here, if the value of the detected current is positive, a compensating voltage V.sub.D for compensating a voltage error according to a dead time is set as a predetermined positive value. If the value of the detected current remains negative, the compensating voltage V.sub.D is set (steps S3-S4) as a predetermined negative value.
Next, by adding a command voltage Vas* to the compensating voltage V.sub.D, there is obtained (step S5) a new command voltage Vas*.sub.-- new.
As shown in FIG. 4 showing a detailed composition view of the control unit 4, a coder 41 judges whether polarity of an output current i1 in the inverter 2, that is, an input current i.sub.as is positive or negative, and if judged positive a positive value is outputted, and then the positive value is multiplied at a multiplier 43 by a dead time compensating voltage Vdead which is added to a command voltage Vas* at an adder 42 to be thereby outputted outside the control unit 4. The outputted voltage denotes a command voltage Vas*.sub.-- new and is applied to the pulse width modulator 5 in FIG. 1, and the pulse width modulator 5 carries out a pulse width modulation of the command voltage Vas*.sub.-- new and outputs control signals CS1-CS6 for controlling switching devices Q1-Q6.
Whereas, when the polarity of current i.sub.as is judged as negative, the command voltage Vas*.sub.-- new is obtained by subtracting dead time compensating voltage Vdead from command voltage Vas* of the inverter.
As described above, conventionally there has been only considered a polarity variation resulting from adding/subtracting dead time compensating voltage Vdead serving as an offset voltage to/from command voltage Vas*, by judging the polarity of current.
However, a dead time compensating voltage is instantly shifted from negative to positive or vice versa so that current is distorted instead of being sinusoidal, thereby generating torque ripple in accordance therewith.
Further, when a motor is in a lower speed mode, the distortion of current wave causes the torque ripple to become larger.